Process and apparatus for forming a plastic sheet having recessed and protruded parts and double wall sheet having the plastic sheet

ABSTRACT

A plastic sheet having a number of recessed and protruded parts useful as a protective sheet in moving and construction, and for cushion packaging and thermal insulation is disclosed. Also disclosed is a two-layered or three-layered double wall sheet, which is produced by adhering one or two flat plastic sheets on the above plastic sheet. The double wall sheet has high rigidity against bending and improved warps when compared with the conventional plastic bubble boards, and is useful as the material for various panels and packaging containers. The plastic sheet is produced by vacuum forming to form plural protruded, truncated cone-shaped parts and recessed, ring-shaped parts surrounding and continuing from the protruding parts. The double wall sheet is produced by adhering a back sheet and/or liner sheet to the plastic sheet.

BACKGROUND OF THE INVENTION

The present invention concerns a plastic sheet having a number ofrecessed and protruded parts prepared by forming recessed parts andprotruded parts on a flat sheet. The invention also concerns atwo-layered or three-layered double wall sheet prepared by combining theabove plastic sheet with another flat plastic sheet or sheets. Theinvention further concerns a method of producing the above-mentionedplastic sheet having a number of recessed and protruded parts, a methodof producing the above-mentioned double wall sheet, and an apparatus forproducing them.

There has been known processed article of thermoplastic material called“plastic bubble sheet”, which is produced with a thermoplastic materialsuch as polyethylene and polypropylene by thermal processing (usually,vacuum forming) to form a cap-film having a number of cap-shapedprojections and by adhering a flat back film to the base surface of thecap film or adhering a flat liner film to the tops of the caps, oradhering both the back film and the liner film. The product is widelyused as the material for cushion-packaging, thermal insulators, or aprotective sheet at moving and construction.

There are variations of the plastic bubble sheets in which thickerfilms, which may be more appropriately called sheets, are used as theconstituting material. The products are called “plastic bubble boards”and are also widely used as the building materials, concrete placingpanels, protecting boards, or materials for various containers.

The difference between the plastic bubble sheet and the plastic bubbleboard resides in the difference of thickness of the films or sheets asthe components, and the difference of thickness of the product plasticbubble sheet and the plastic bubble board, on which the strength and therigidity of the products depend, and therefore, the two kinds ofproducts are applied to the appropriate use. In the followingdescription the material films and sheets are represented by a “sheet”,and the product bubble sheets and bubble boards, by a “bubble board”.

The above described bubble board have two grades: the two-layered bubbleboard prepared by adhering a back sheet to the base of the cap sheet orby adhering a liner sheet to the tops of the caps, and the three-layeredbubble board prepared by adhering both the back sheet and the linersheet. Of the two-layered ones, those having a back sheet have lowerrigidity, and therefore, are applied to the use where no bendingrigidity is required. Even though the two-layered ones having a linersheet can be applied to the use where the bending rigidity is required,it is a drawback that the products are not suitable as the protectingsheet, because those who stepped on the sheet may miss their footing.The three-layered ones have the highest bending rigidity and both thesurfaces are smooth, and therefore, are suitable for applying to the usewhere such the features are required and where slipping is out of thequestion. The weak point of the three-layered bubble boards is, due tothe producing process, that warps tend to occur during producing.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide a plastic sheethaving a number of recessed and protruded parts with some cushioningability to the stress in the direction vertical to the face of thesheet, and when used as the protecting sheet, exhibiting good non-slipproperty.

The second object of the present invention is to provide a two-layeredor three-layered double wall sheet having the above plastic sheet as thecomponent, to which one or two flat plastic sheet are adhered, andexhibiting higher bending rigidity and improved warp when compared withthe conventional plastic bubble boards.

The object of the invention includes providing a method of producing theplastic sheet having a number of recessed and protruded parts, a methodof producing the double wall sheet having the above plastic sheet as thecomponent, as well as an apparatus for carrying out the methods ofproducing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an embodiment of a plasticsheet having a number of recessed and protruded parts according to theinvention;

FIG. 2 is a perspective view, like FIG. 1, illustrating anotherembodiment of a plastic sheet having a number of recessed and protrudedparts according to the invention;

FIG. 3 is a perspective view, like FIG. 1, illustrating a furtherembodiment of a plastic sheet having a number of recessed and protrudedparts according to the invention;

FIG. 4 is a perspective view, like FIG. 1, illustrating a still otherembodiment of a plastic sheet having a number of recessed and protrudedparts according to the invention;

FIG. 5 is a perspective view, like FIG. 1, illustrating a still furtherembodiment of a plastic sheet having a number of recessed and protrudedparts according to the invention;

FIG. 6 is a perspective view illustrating an embodiment of a two-layereddouble wall sheet of the invention;

FIG. 7 is a perspective view, like FIG. 6, illustrating anotherembodiment of a two-layered double wall sheet of the invention;

FIG. 8 is a perspective view, like FIG. 6, illustrating still otherembodiment of a two-layered double wall sheet of the invention;

FIG. 9 is a perspective view illustrating an embodiment of athree-layered double wall sheet of the invention;

FIG. 10 is a conceptional side elevation view illustrating essentialparts of an apparatus for producing the plastic sheet having a number ofrecessed and protruded parts according to the invention;

FIG. 11 is a conceptional side elevation view illustrating essentialparts of an apparatus to explain a method of producing the two-layereddouble wall sheet of the embodiments of the invention as shown in FIGS.1 to 6;

FIG. 12 is a conceptional side elevation view illustrating essentialparts of an apparatus to explain another method of producing thetwo-layered double wall sheet of the embodiments of the invention asshown in FIGS. 1 to 6;

FIG. 13 is a partial, longitudinal section view illustrating structureof a vacuum-forming roll used in an embodiment of apparatus forproducing a plastic sheet having a number of recessed and protrudedparts according to the invention;

FIG. 14 is a perspective view showing the shape of a forming-plug usedin the vacuum-forming roll of FIG. 13;

FIG. 15 is a partial, longitudinal section view, corresponding to FIG.13, illustrating structure of a vacuum-forming roll used in anotherembodiment of apparatus for producing a plastic sheet having a number ofrecessed and protruded parts according to the invention;

FIG. 16 is a perspective view showing the shape of a forming-plug usedin the vacuum-forming roll of FIG. 15; and

FIG. 17 is a section view of the forming-plug shown in FIG. 16 toexplain a preferred fixation of the plug.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The plastic sheet according to the present invention is, as shown inFIG. 1, a plastic sheet (1) having a number of recessed and protrudedparts produced by thermal processing to form a number of uneven partsconsisting of a protruding, truncated cone-shaped part (12) and arecessed, ring-shaped part (13) surrounding and continuing from theprotruding part on a plane (11) of a flat plastic sheet.

The plastic sheet having a number of recessed and protruded partsaccording to the invention includes various embodiments. In theembodiment shown in FIG. 1, the height (H) of the protruding, truncatedcone-shaped part (12) from the bottom of the cone is larger than thedepth (D) of the recessed, ring-shaped part (13), or H>D, which is thetypical embodiment.

This embodiment includes the following three types:

-   a) H>2D The embodiment in which the height (H) of projection of the    truncated cone-shaped part from the plane is larger than the    depth (D) of the recessed, ring-shaped parts. This is shown in FIG.    2.-   b) H=2D The embodiment in which the height (H) of projection of the    truncated cone-shaped part from the plane is the same as the    depth (D) of the recessed, ring-shaped parts. This is shown in FIG.    1.-   c) H<D<2H The embodiment in which the height (H) of projection of    the truncated cone-shaped part from the plane is smaller than the    depth (D) of the recessed, ring-shaped parts. This is shown in FIG.    3.

Further embodiments as follows are possible.

-   d) H=D The embodiment in which the height (H) of projection of the    truncated cone-shaped part based on the bottom of the recessed,    ring-shaped part is nearly the same as the depth (D) of the    recessed, ring-shaped parts. This is shown in FIG. 4.-   e) H<D The embodiment in which the height (H) of projection of the    truncated cone-shaped part based on the bottom of the recessed,    ring-shaped part is smaller than the depth (D) of the recessed    ring-shaped parts. This is shown in FIG. 5.

The two-layered double wall sheet according to the present inventionalso includes various embodiments. One of them is the two-layered doublewall sheet (2A) shown in FIG. 6, which uses the plastic sheet (1) havinga number of recessed and protruded parts, wherein H>D, as the componentand a liner sheet is adhered to the tops of the protruded, cone-shapedpart (12).

Another one is the two-layered double wall sheet (2B) shown in FIG. 7,which uses the plastic sheet (1) having a number of recessed andprotruded parts, wherein H=D, as the component, and a liner sheet (5) isadhered to the tops of the protruded, cone-shaped parts (12) and theplane (11). In case where the component plastic sheet is such one asH=D, then the product double wall sheet (not illustrated) will have theflat liner sheet only on the plane (11).

A further one is the two-layered double wall sheet (2C) shown in FIG. 8,which uses the plastic sheet (1) having a number of recessed andprotruded parts as the component, and a flat back sheet (4) is adheredto the bottoms of the recessed, ring-shaped parts (13).

The three-layered double wall sheet (3) of the invention is produced byadhering a flat liner sheet (5) or flat back sheet (4) to any one of theabove-described double wall sheets (2A, 2B or 2C) on the side oppositeto the side on which a back sheet or a liner sheet is already adhered.This product may be understood either as such one that a flat back sheet(4) is adhered to the above-described two-layered double wall sheet (2Aor 2B) having a liner sheet (5), or such one that a flat liner sheet (5)is adhered to the above-describe two-layered double wall sheet (2C)having a back sheet (4). FIG. 9 shows an embodiment of this product.

For the purpose of meeting elevated demand for the bending rigidity andthe plane compressive strength, even in the conventional bubble boards,there has been such a product as “double ply-type”, which uses a pair ofcap sheets and flat sheets are disposed between the two cap sheets andon both the front and the back sides. In the double wall sheet accordingto the invention the plane part of the plastic sheet having a number ofrecessed and protruded parts takes the roll of the flat sheet betweenthe two cap sheets of the conventional products, and thus, even thetwo-layered structure exhibits performance equal to that of thethree-layered product. It can be easily understood that thethree-layered product of the double wall sheet of the invention hasperformance exceeding that of the conventional double ply-type bubbleboard.

The method of producing the plastic sheet according to the invention asshown in FIG. 1 comprises the steps of: feeding a plastic sheet in themolten state extruded from a T-die (8) to a vacuum forming roll (6),which rotates around the axis at the center, and has plural projectionsand ring-shaped recesses each surrounding the projections and vacuumsuction passages therein; forming plural protruding, truncatedcone-shaped parts and recessed, ring-shaped part surrounding andcontinuing from the protruding part to have the parts evenly distributedon a plane of the plastic sheet, and pealing the formed plastic sheetfrom the vacuum forming roll.

A method of producing the two-layered double wall sheet (2A, 2B and 2C)as shown in FIGS. 6 to 8 comprises, further to the above-describedsteps, as FIG. 11 illustrates the essential parts of the apparatus forproducing, the steps of: feeding another flat plastic sheet in themolten state extruded from another T-die to the tops of the pluraltruncated cone-shaped parts or to the flat surface of the sheet so as toadhere by fusion to give the liner sheet (5), or to the bottoms of therecessed, ring-shaped parts so as to have the flat sheet adhered byfusion to give the back sheet (4) to the plastic sheet. In FIG. 11,numerical reference “9” shows a pressing roll for adhering the linersheet to the plastic sheet.

Another method of producing the two-layered double wall sheet (2A, 2Band 2C) as shown in FIGS. 6 to 8 comprises, following to theabove-described steps for producing the plastic sheet having a number ofrecessed and protruded parts, as FIG. 12 illustrates the essential partsof the apparatus for producing, the steps of: feeding another flatplastic sheet in the molten state extruded from another T-die (8) to thebottoms of the plural recessed, ring-shaped parts of the plastic sheet,which was pealed out from the vacuum forming roll so as to adhere byfusion to give the back sheet (4), or to the tops of the pluraltruncated cone-shaped parts so as to have the flat sheet adhered byfusion to give the liner sheet (5). Also in FIG. 12, numerical reference“9” shows a pressing roll for adhering the back sheet.

The method of producing the three-layered double wall sheet (3) of theinvention illustrated in FIG. 9 comprises, as may be understood from theabove explanation, carrying out both the above-described additional andsupplemental steps. In other words, the liner sheet or the back sheet isgiven to the plastic sheet having a number of recessed and protrudedparts during producing the plastic sheet, and then, the remaining backsheet or the remaining liner sheet is given to the plastic sheet.

The apparatus according to the invention for producing the plastic sheethaving a number of recessed and protruded parts has no principaldifference from the conventional apparatus for producing plastic bubbleboards. As shown in FIG. 13 and FIG. 15, the apparatus comprises avacuum forming roll cylinder (61) made of metal, which is supportedrotatably around the axis (62) thereof, in which vacuum suction channels(63) are provided; and forming plugs (7) having vacuum suction passages,which are fixed on the surface of the cylinder so that the tops of theplugs may be higher than the surface of the cylinder. The vacuum systemis so regulated that the plugs are connected to the vacuum source (notshown) only when they are in a certain rotational position. Theapparatus of the invention may be constructed by remodeling of theexisting apparatus.

The forming plug (7) to be fixed on the forming roll cylinder (6) isroughly classified into two types. FIG. 14 shows one (7A) of the twotypes. The plug of this type comprises: a head (71A), which is apositive projection of the shape corresponding to the protruded,truncated cone-shaped part to give the shape to the extruded flatplastic sheet, and a base (72A) with threads (73) for fixing the plug tothe forming roll cylinder. The plug (7A) forms the negative parttogether with the recessed part provided on the surface of the formingroll cylinder, which gives the recessed, ring-shaped part to the flatplastic sheet. The forming plug (7A) has vacuum suction grooves (74)around the base. Fixation of the plugs to the forming roll cylinder (61)may be done by screw clamping using the threads as illustrated, and thisway is superior in view of sure fixing and possibility of replacement ofthe plugs. However, in case where necessity of replacing the formingplugs is low, it will be enough to fix the plugs by striking them to fitin the forming roll cylinder.

Various embodiments are possible in the forming plug (7A). Of course theprojection may not necessary be truncated cone shape with round sectionbut may be of a pyramid shape with hexagonal or octagonal section. Itmay be possible that the top of the projection is round and the base ispyramidal. Such a shape can be compared to the head of a pencil on theway of sharpening. Anyway, the plug having a pyramidal part can beturned with a wrench. In case of using the plugs of round cone withoutany hold for wrench, it is recommended to provide a plus or minus grooveat the top of the plug so that the plug may be turned with a driver.Preparation of the forming plugs can be carried out by machining, andthis is an easy way of manufacturing. However, because of plurality ofthe plugs necessary for one vacuum forming roll, it is recommended toemploy a more efficient way of manufacturing such as formation of theheads by closed die forging followed by thread rolling.

Another embodiment (7B) of the forming plug is shown in FIG. 16 and FIG.17. The plug of this type comprises: a head (71B), which has a negativerecess (75) of the shape corresponding to the protruding part and apositive projection (76) of the shape corresponding to the recessed partsurrounding the negative recess to give the ring-shaped recess to theextruded flat plastic sheet, and a base (72B) with threads for fixingthe plug on the forming roll cylinder (6). The forming plug (7B) alsohas a vacuum suction hole at the bottom of the negative recess and thevacuum suction grooves (77) around the positive projection of the base.

The above description on the configuration and manufacturing concerningthe first embodiment (7A) of the forming plug (7) is applicable to thesecond embodiment (7B) as it is. However, in case where the secondembodiment is chosen, if the diameter of the vacuum forming rollcylinder is relatively small and the forming plug is large, simplefitting by screw clamping causes, due to the curvature of the formingroll cylinder, formation of gaps between the forming plugs and theforming roll cylinder at some parts, and care must be taken for this. Atsuch a part the heat possessed by the plastic sheet in the molten statewill not be sufficiently transmitted to the cylinder. Then, accumulationof heat will occur in the forming plugs and results in temperatureincrease of the plugs. It is needless to say that local temperatureincrease in the apparatus is unfavorable to plastic processing.

Such problem of heat accumulation may be solved by entire contact of theforming plugs to the forming roll cylinder. It is advisable, as shown inFIG. 17, to cut down the surface of the cylinder on which the formingplug is fixed.

The plastic sheet having a number of recessed and protruded partsaccording to the present invention is relatively soft and has, inaddition to good cushioning property, excellent ability in preventingslip. Thus, the plastic sheet of the invention is particularly suitableas a simple protecting sheet. The two-layered double wall sheet made bycombining this plastic sheet and a back sheet or a liner sheet hasextremely higher bending rigidity and plane compressive strength whencompared with conventional plastic bubble boards, and therefore, it isuseful as the material for various panels. Relatively thin products ofthe two-layered double wall sheet has, though the bending rigidity andthe plane compressive strength are not high, better cushioning property,and is useful as the material for various cushion packaging.

The three-layered double wall sheet made by combining the plastic sheethaving a number of recessed and protruded parts with both the back sheetand the liner sheet exhibits remarkably high bending rigidity and planecompressive strength, and the warp as manufactured is much better thanthat of the conventional products. This is suitable as the material fora packaging container to contain heavy things.

The double wall sheet according to the invention, of course thethree-layered product, even the two-layered product, is characterized byits high thermal insulating property. This seems to be caused by thefact that movement of air confined in the bubbles is more stronglyprevented in the present product when compared with the conventionalplastic bubble boards due to the structural difference. It is knownthat, in the bubble boards, as the size of the bubble reaches a certainextent, convection of the air occurs in the bubbles and results indecrease of the thermal insulating effect. However, in the double wallsheet of the invention, the space in the protruded, cone-shaped partsand the space in the recessed, ring-shaped parts are divided by theplastic sheet, the convection of air in question can occur only in theseparate spaces. Thus, the heat transfer by convection is reduced.

EXAMPLES Example 1 Plastic Sheet Having a Number of Recessed andProtruded Parts

A polyethylene of density 0.92 g/cm³ and MI 2 g/10 min. was used as thematerial and a plastic sheet having a number of recessed and theprotruded parts was produced with the following specification. Theproduct was suitable for the protecting sheet.

Thickness of the plane part: 0.5 mm

Height of the protruded parts H: 10 mm

Diameter of the truncated cone: 7 mm

Depth of the recessed part D: 10 mm

Diameter of the recessed part: 32 mm

Pitch: 55 mm

Distribution: staggered

Example 2 Two-Layered Double Wall Sheet

On the plastic sheet having a number of recessed and protruded partsproduced in Example 1, a sheet of the same material of the thickness 0.3mm was laminated as the back sheet to produce a two-layered double wallsheet of the invention. The double two-layered wall sheet had thefollowing properties:

-   Bending rigidity: 35 Kg (measured by the bending test according to    JIS Z7204)    Plane compressive strength: 25 Kgf/cm²

Example 3 Three-Layered Double Wall Sheet

On the plastic sheet having a number of recessed and protruded partsproduced in Example 1, two sheets of the same material of the thickness0.3 mm were laminated as the back sheet and the liner sheet to produce athree-layered double wall sheet of the invention. The three-layereddouble wall sheet had the following properties:

Bending rigidity: 55 Kg (measured as above)

Plane compressive strength: 27 Kgf/cm².

1. A method of producing the plastic sheet having a number of recessedand protruded parts produced by thermal processing to form a number ofuneven parts consisting of a protruding, truncated cone-shaped part anda recessed, ring-shaped part surrounded and continuing from theprotruding part on a plane of a plastic sheet, comprising the steps of:feeding a plastic sheet in the molten state extruded from a T-die to avacuum forming roll, which rotates around an axis at a center of thevacuum forming roll and has plural projections and ring-shaped recesseseach surrounding the projections and vacuum suction passages therein;forming plural protruded, truncated cone-shaped parts and recessed,ring-shaped parts surrounding and continuing from the protruding partsto have the parts evenly distributed on the plane of the plastic sheet,and peeling the formed plastic sheet from the vacuum forming roll.
 2. Amethod of producing the two-layered double wall sheet produced byadhering a flat liner sheet to tops of the truncated cones of theplastic sheet having a number of recessed and protruded parts producedby thermal processing to form a number of uneven parts consisting of aprotruding, truncated cone-shaped part and a recessed, ring-shaped partsurrounded and continuing from the protruding part on a plane of aplastic sheet, comprising the steps of: feeding a first plastic sheet inthe molten state extruded from a T-die to a vacuum forming roll, whichrotates around an axis at a center of the vacuum forming roll and hasplural projections and ring-shaped recesses each surrounding theprojections and vacuum suction passages therein; forming pluralprotruded, truncated cone-shaped parts and recessed, ring-shaped partssurrounding and continuing from the protruding parts to have the partsevenly distributed on the plane of the first plastic sheet; peeling theformed first plastic sheet from the vacuum forming roll; and feeding asecond plastic sheet in the molten state extruded from another T-die tothe tops of the plural truncated cone-shaped parts or to a flat surfaceof the sheet so as to adhere by fusion to give the liner sheet, or tobottoms of the recessed, ring-shaped parts so as to have the secondsheet adhered by fusion to give the back sheet to the first plasticsheet.
 3. A method of producing the two-layered double wall sheetproduced by adhering a flat back sheet to tops of the truncated cones ofthe plastic sheet having a number of recessed and protruded partsproduced by thermal processing to form a number of uneven partsconsisting of a protruding, truncated cone-shaped part and a recessed,ring-shaped part surrounded and continuing from the protruding part on aplane of a plastic sheet, comprising the steps of: feeding a firstplastic sheet in the molten state extruded from a T-die to a vacuumforming roll, which rotates around an axis at a center of the vacuumforming roll and has plural projections and ring-shaped recesses eachsurrounding the projections and vacuum suction passages therein; formingplural protruded, truncated cone-shaped parts and recessed, ring-shapedparts surrounding and continuing from the protruding parts to have theparts evenly distributed on the plane of the first plastic sheet;peeling the formed first plastic sheet from the vacuum forming roll; andfeeding a second plastic sheet in the molten state extruded from anotherT-die to bottoms of the plural recessed, ring-shaped parts of the firstplastic sheet pealed from the vacuum forming roll so as to adhere byfusion to give the back sheet, or to the tops of the plural truncatedcone-shaped parts or to a flat surface of the first plastic sheet so asto have the second plastic sheet adhered by fusion to give the linersheet.
 4. A method of producing the three-layered double wall sheetproduced by adhering a flat liner sheet to tops of the truncated conesand a flat back sheet to the bottom of the recessed, ring-shaped part ofthe plastic sheet having a number of recessed and protruded partsproduced by thermal processing to form a number of uneven partsconsisting of a protruding, truncated cone-shaped part and a recessed,ring-shaped part surrounded and continuing from the protruding part on aplane of a plastic sheet, comprising the steps of: feeding a firstplastic sheet in the molten state extruded from a first T-die to avacuum forming roll, which rotates around an axis at a center of thevacuum forming roll and has plural projections and ring-shaped recesseseach surrounding the projections and vacuum suction passages therein;forming plural protruded, truncated cone-shaped parts and recessed,ring-shaped parts surrounding and continuing from the protruding partsto have the parts evenly distributed on the plane of the first plasticsheet, peeling the formed first plastic sheet from the vacuum formingroll; and feeding a second plastic sheet in the molten state extrudedfrom a second T-die to the tops of the plural truncated cone-shapedparts or to a flat surface of the first plastic sheet so as to adhere byfusion to give the liner sheet, or to bottoms of the recessed,ring-shaped parts so as to have the second sheet adhered by fusion togive the back sheet to the first plastic sheet; and feeding a thirdplastic sheet in the molten state extruded from a third T-die to thebottoms of the plural recessed, ring-shaped parts of the third plasticsheet pealed from the vacuum forming roll so as to adhere by fusion togive the back sheet, or to the tops of the plural truncated cone-shapedparts or to the flat surface of the first plastic sheet so as to havethe third plastic sheet adhered by fusion to give the liner sheet.
 5. Anapparatus for carrying out the method of producing the plastic sheethaving a number of recessed and protruded parts defined in claim 1,comprising: a vacuum forming roll cylinder made of metal, which issupported rotatably around the axis thereof, and inside of which isconnected to a vacuum source; forming plugs having vacuum suctionpassages and fixed on the surface of the forming roll cylinder so thatthe tops of the plugs may be higher than the surface of the cylinder;and a vacuum system allowing connection of the plugs to the vacuumsource only when the plugs are in a certain rotational position.
 6. Theapparatus according to claim 5, wherein each forming plug includes ahead, which is a positive projection of the shape corresponding to theprotruded, truncated cone-shaped part, and a base with threads forfixing the plug to the forming roll cylinder, each plug forming anegative part together with the recessed part provided on the surface ofthe forming roll cylinder to give the ring-shaped recess, and havingvacuum suction grooves around the base.
 7. The apparatus according toclaim 5, wherein each forming plug includes a head, which has a negativerecess of the shape corresponding to the protruding part and a positiveprojection of the shape corresponding to the recessed part surroundingthe negative recess to give the ring-shaped recess to the flat plasticsheet, and a base with threads for fixing the plug on the forming rollcylinder, each plug having a vacuum suction hole at the bottom of thenegative recess and the vacuum suction grooves (77) around the positiveprojection of the base.